1. Field of the Invention
The present invention relates to a mammography apparatus for obtaining a breast image.
2. Description of the Related Art
Breast cancer screenings are conducted using X-ray machines dedicated to breast imaging (mammography apparatus), since the discovery rate of early stage cancer is increased by combining mammograms obtained by the mammography apparatus with clinical breast examinations rather than depending only on the clinical breast examinations.
General breast cancer screenings include a unidirectional imaging (two images) and a two-directional imaging (four images). In the unidirectional imaging, only the MLO (mediolateral oblique) imaging is conducted. In the MLO imaging, the breast is clamped and imaged from the oblique direction so that the greater pectoral muscles are imaged as shown in FIG. 14A. In order to enable the imaging from such direction, the mammography apparatus includes an arm to which a radiation irradiation section and an object table are connected such that they face each other, and the object table is inclined by rotating the arm. In the two-directional imaging, both the MLO imaging and CC (craniocaudal) imaging are conducted. In the CC imaging, the breast is clamped and imaged from above as shown in FIG. 14B. The MLO imaging may provide an image having the widest area of a breast. The CC imaging is performed to complement the MLO imaging, since it may visualize inside areas of a breast which are difficult for the MLO imaging to visualize as an image. Generally, a unidirectional imaging and a two-directional imaging is performed for each of the right and left breasts, and two images are obtained in the unidirectional imaging and four images are obtained in the two-directional imaging.
The breast includes mammary gland tissues, fat tissues and skin. In order to obtain a breast image required for giving an accurate diagnosis, the breast needs to be compressed evenly as much as possible to image small mammary glands and fat tissues on a film. Therefore, in the mammography apparatus, the breast is imaged while it is clamped between the object table and a compression paddle. If the pressing pressure is too low, however, the mammary glands, fat, blood vessels and the like overlap with each other. Consequently, there may be a case in which a tumor actually existing in the breast is not imaged. Thus, in order to obtain a good image, the breast needs to be compressed to a certain extent when imaged. That is, the pressing pressure needs to be controlled such the thickness of the breast becomes appropriate for imaging for each type of breast.
Further, AEC (automatic exposure control) for measuring the dose of radiation transmitted through the breast is essential in order to stably obtain an appropriate film density by the mammography apparatus. The AEC is a sensor provided under the cassette, and in general, semiconductor detectors are disposed as the sensor. When imaging a breast of a subject, the position of the breast and the like are adjusted by the operator so that the dose of radiation irradiated on the center of the breast is measured by the sensor.
Therefore, when the two-directional imaging is performed for the right and left breasts of a subject, such adjustments are required to be made four times, causing a considerable burden on the subject.
Consequently, a mammography apparatus, in which an initially adjusted pressing pressure value or the like for each subject is stored for repeatedly performed mammography screening, and imaging is performed using the stored value from the next time, is proposed as described, for example, in International Patent Publication No. WO01/054463.
Further, in order to obtain an image having an optimum density, it is necessary to place the breast on the object table that includes AEC such that the center of the breast corresponds to the center of the object table so that the dose of X-rays transmitted through the central portion of the breast is detected. The size of the breast, however, varies greatly between individuals, so that a large object table is used for imaging a large breast, and a small object table is used for imaging a small breast so that the breast is placed on the center of the object table in the apparatus proposed in the aforementioned patent publication.
The burden on the subject may be alleviated by storing information related to the pressing pressure and the like as proposed in the aforementioned patent publication, but it is difficult to invariably place the breast on the center of the object table. Consequently, there may be a case that the density of the image is inappropriate.
Further, the size of the breast varies greatly between individuals, and if a small breast is imaged using a large object table, the position of the breast may be displaced from the center of the object table, so that it is desirable to use object tables of different sizes according to the size of the breast. But, the object table having a flat panel detector accommodated therein is expensive, so that it may be difficult to invariably provide both the large and small platforms.
Consequently, when a small breast is imaged using a large object table, the dose of radiation detected by the AEC sensor is not always the dose of radiation transmitted through the central portion of the breast. This may result in the image having an inappropriate density.
Further, use of a large platform for imaging a small breast causes a problem that it is hard for the operator to withdraw its hand after positioning the breast.
It is an object of the present invention, therefore, to provide a mammography apparatus capable of automatically obtaining a breast image having an optimum density with reduced burden on the subject.